This article will explore seven basic (and critical) techniques and strategies for both novice and experienced printed circuit board (PCB) design. As long as you pay more attention to these skills in the design process, you and your team can reduce redesigns. The number of times, shorten the design time and reduce the overall design result diagnosis task; let us look at one by one below.
1. Familiar with the factory manufacturing process. In this era of fabless IC industry, many engineers are actually not clear about the PCB production steps and chemical treatment processes based on their design files; this is not surprising. However, this lack of practical knowledge often leads novice engineers to make unnecessary and more complex design decisions. Does the design really need to be that complicated? Isn't it possible to use a larger grid for wiring, thereby reducing the cost of the circuit board and improving reliability? Other mistakes that design novices are likely to make are unnecessary small via sizes, blind vias and buried vias. Those advanced through-hole structures are a weapon for PCB designers, but their effectiveness is highly contextualized. Although they are usable tools, they do not mean that they must be used. A blog post by PCB design expert Bert Simonovich talked about the issue of through-hole size ratio: "Through-holes with an aspect ratio of 6:1 can ensure that your circuit board can be produced anywhere." For most designs With a little thought and planning, you can avoid those high-density (HDI) features and again save costs and improve the manufacturability of the design. Those ultra-small size or single-ended (dead-ended) through-hole copper plating requires physical and fluid dynamics capabilities, not all PCB foundries are good at. Remember, as long as there is a bad via can destroy the entire circuit board; if your design has 20,000 vias, then you have 20,000 chances of failure. Unnecessarily using HDI vias, the failure rate immediately soared.
2. Circuit diagrams can simplify design tasks. Sometimes just designing a simple circuit board, drawing schematics seems to be a waste of time; especially if you have experience in completing one or two designs. But for people who are designing PCBs for the first time, drawing circuit diagrams can also be a difficult task. Skip the circuit diagram is a strategy often adopted by novices and design engineers with moderate experience, but please develop your wiring from a complete circuit diagram that can be used as a reference to help ensure that your wiring connections can be completed. ; The following is the reason. First of all, the circuit diagram is the visual presentation of the PCB circuit, which can convey multiple levels of information; the sub-regions of the circuit are drawn in detail on several pages, and the components corresponding to the functions can be arranged in adjacent positions, regardless of the final physical layout. Secondly, because the circuit diagram symbol will mark each pin of each component, it is easy to check the disconnected pins; in other words, regardless of whether the formal rules describing the circuit are followed, the circuit diagram helps you quickly visualize Determine to ensure the integrity of the circuit. When designing the PCB, if there is a circuit diagram that can be used as a basic template, it can simplify the wiring task. Use the circuit diagram symbols to complete the link, and at the same time you will overcome the wiring challenge without having to think about those connections; in the end, you will save the design rework because you have caught the wiring connections that were missed in the first revision.
3. Use automatic routers but don’t rely on most professional PCB CAD tools that have automatic routers, but unless you design the PCB very professionally, automatic routers can only be used to make the design preliminary at best; for PCB circuit links , Autorouter is not a one-click solution, you should still know how to route it manually. Auto-router is a highly configurable tool. In order to give full play to their role, router parameters must be carefully and thoughtfully set for each task, and even individual modules on a single PCB must be considered individually. In short, there is no proper basic universal default value. When you ask an experienced design engineer: "Which auto-router is best to use?" They will answer: "The thing between the ears (eyes);" And they are serious. The process of wiring is more like an art like an algorithm. It is heuristic in itself, so it is very similar to the traditional backtracking algorithm. The backtracking algorithm is very suitable for finding solutions, especially in situations where path choices such as mazes or puzzles are restricted; but in an open and unrestricted occasion, such as a PCB with pre-placed components, the backtracking algorithm cannot be used to find optimization. The strengths of the solution. Unless the constraints of the automatic router are carefully fine-tuned by the engineer, the finished wiring still needs to be manually checked for weaknesses in the results of the backtracking algorithm. The size of the trace is another problem. The auto-router cannot be 100% sure of how much current you intend to pass on a trace, so it cannot help you determine how wide the trace is to be used; the result is that most auto-routers produce traces. The line width does not meet the specifications. When you consider using an auto-router, ask yourself: “After I set the auto-router constraints for the board, and even set the constraints for each trace on the circuit diagram, how much time do I have? Manual wiring?" Veteran design engineers will focus most of their energy on the initial part layout. Almost half of the entire design time is devoted to optimizing the component layout from the following three aspects:
Simplified wiring-minimize the crossing of flying wires (rat's nest, or rat's nest, rat's nest) and so on.
Proximity of components-the shorter the winding, the better.
Signal timing considerations.
The elders often use a mixed method to route-use manual key routing, fix their location, and then use the automatic router to process non-critical routing; the automatic routing area in the design helps to manage the "out of control" in the routing algorithm. runaway) state", this method can sometimes achieve a good compromise between the controllability of manual wiring and the speed of automatic wiring. 4. Consider circuit board size and current. Most people who are engaged in electronic design know that, just like a river walking along a river, flowing electrons may also encounter throat points and bottlenecks; this is directly applied to car fuses ( Automotive fuse) is being designed. By controlling the thickness and shape of the trace (U-shaped bending, V-shaped bending, S-shaped, etc.), the fuse can be calibrated to blow at the throat point when the current is overloaded. The problem is that design engineers occasionally encounter similar electrical throat points in their PCB designs; for example: where two steep 45 degrees can also be used, use a 90-degree bend; when the bend is greater than 90 degrees, Adopt a zigzag shape. At best, those wires will only slow down the signal propagation; at worst, they will blow at the resistance point like a car fuse.
5. Avoiding the risk of slivers Slivers are a manufacturing error that can be best managed through proper circuit board design (see Figure 1); in order to understand the sliver problem, we need to review the chemical etching process. Chemical etching is to decompose unnecessary copper, but if the part to be etched is particularly long, thin, and flakes, those shapes will sometimes be peeled off before being completely decomposed; such lobes will float in the chemical solution. It may fall randomly on another circuit board.
Figure 1 In this case, the narrow shield between the traces is safe for the circuit board.
It is also possible that the risk is that the split remains on the original circuit board; if the split is narrow enough, the acid pool may corrode enough copper below to partially peel off the split. As a result, the slivers stuck to the circuit board like flags and floated around, and in the end they inevitably fell on the board and caused other traces to short-circuit. So where to look for potential splinters? How to avoid splinters? When wiring the PCB, it is best to avoid leaving a very narrow copper wire area (Figure 2); this area is usually caused by the intersection of the trace and the pad gap and the plane being filled (Figure 3). Set the minimum width of the copper wire to be greater than the minimum allowed by the manufacturer, and your design should not have this problem. The standard minimum etching width is 0.006 inches.
Figure 2 A very narrow risk area for splitting, such as the case in the original design file in the figure, may be uncontrollably peeled off during manufacturing, causing short circuits and yield problems.
Figure 3 In this case, chemical etching will change the shape/size of the narrow lobes filling; when the lobes peel off, unexpected debris or floating objects will be generated.
6. Pay attention to the setting of DRC auto-router usually for design functions, and design rule checker (Design Rule Checker, DRC) is generally used to capture the manufacturer’s design constraints; although the setting process is also cumbersome, compared with auto-router It looks much better. Most design teams will eventually establish a set of design rules with the goal of standardizing bare board production costs, maximizing yield, and making assembly, inspection, and testing as consistent as possible. In addition to benefiting the design, these design rules—by maintaining the design within predefined manufacturing limits—also help to establish consistency in the purchasing department; if the price of the circuit board manufacturing is consistent, the purchase is usually Can reduce the number of specific PCB manufacturing agreements that need to be maintained. In order to solve all these problems, many PCB design tools have built-in DRC-some tools call them "constraint managers"-when you are editing, DRC will interactively mark design violations; and Once you have set the DRC rules for the manufacturer of your choice, be prepared to take mistakes seriously. DRC tools are generally conservative in design. They also make mistakes when reporting possible errors, which must be judged by you; screening hundreds of "possible" problems can be tedious, but you have to do it anyway. In this list of questions, there may be reasons why your first tapeout is doomed to fail. In addition, if your design triggers a large number of possible errors, it means that your wiring needs to be improved. Dave Baker, a circuit board design engineer at Sunstone Circuits with more than two decades of experience, suggested: "Take time to understand and correctly set up the constraint system provided by the wiring tool, and review all levels of constraints; the constraint tool can be very powerful and flexible Yes, but it can also be confusing and dangerous. Wrong constraints can easily lead to defective or unmanufactured circuit boards. Errors in the constraint settings are likely to limit DRC or make it inoperable." He pointed out for example: "Maybe What happened was that every time the DRC passed, but the circuit board still could not be manufactured or had no function. I have seen this situation before. Originally, the design team was very happy because the circuit board passed the DRC inspection. As a result, the first batch of products were released. It smokes when you go to the test bench; in order to track down the cause of the failure, the team returns to the constraint manager of the CAD tool; the constraint manager has no design awareness, and it will let you do anything, no matter how bad things are.†For example, Sunstone Circuits almost Every day, we receive the circuit board design quotation request that we can easily build, unless it is in some key areas where the design tolerance and gap are greatly compressed. This situation makes PCB foundries (such as Sunstone) have to tell the bad news: because the tolerance is beyond our ability, we can’t make circuit boards; or we can make circuit boards, but there is a price increase and there is a wind of yield. limit. Such customers will benefit from it if they consider the capabilities of a specific manufacturer when designing.
7. Know the existing foundry partners. After discussing the DRC settings, the final PCB design secret is almost-but not completely-redundant; besides helping you set the DRC rules correctly, design your circuit board The factory that is about to be sent to the production understands it and can get some additional assistance before entering the factory. A good foundry will provide some assistance and advice before placing an order, including how to deal with your design to reduce design iterations, reduce problems encountered during debugging on the test bench, and increase the yield of circuit boards. Hugo, a doctoral student at Carnegie-Mellon University in the United States, commented on the issue of understanding manufacturers in a blog post: "Each manufacturer has its own specifications, such as minimum trace width, spacing, and layer. Count and so on; before starting to design, you should consider your requirements, and then find a manufacturer that can meet you. Your needs also include PCB material grades, from FR-1 (paper-phenolic resin mixture) to FR -5 (glass fiber and epoxy resin); most PCB prototype manufacturers use FR-4, but FR-2 is also commonly used in a large number of consumer applications. The type of material affects the strength, durability, moisture absorption and resistance of the circuit board Flammability (FR)." Knowing the PCB process and which process and production method your manufacturer will use will help you make better design decisions; visit your favorite manufacturing service provider and see for yourself Craftsmanship, you may be surprised. Also, make good use of design for manufacturability (DFM) tools before sending the design drawings to production.
Summary If you have mastered the above seven basic skills, it means that you have been on the road to achieve fast, reliable, and professional-quality PCB. Use all the design tools provided by CAD tools wisely, including automatic layout and automatic routing, but you must be patient and careful when setting up the automatic routing device in order to achieve good automatic routing results. Do not rely on the auto-router for anything other than wiring; if it is necessary to ensure that the current in the design is appropriate, manually adjust the trace size. In any case, you must believe in the flying line. Until all these are fully achieved, your circuit design will be successful.
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